The present invention is directed to integrated circuits and, more particularly, to a controller for a persistent display panel.
Persistent display panels are widely used in e-readers, for example. An e-reader is a portable battery-powered electronic device that is particularly adapted to reading text, with or without pictures. An e-reader has good readability in bright ambient light, even sunlight. The power consumption is very low, giving long battery life between recharging. Some e-readers have a battery life of one to two months before recharging.
For displaying video or games, for example, the frame displayed is refreshed continuously at a rate of 50 Hz to 200 Hz or more to display motion pictures. In an e-reader, however, the image of the page displayed is static and does not change while the page is read. Updating continuously the page to be displayed in an e-reader at such a fast frame refresh repetition rate is unnecessary. A persistent display panel is not well suited to displaying motion pictures but is well adapted to displaying the static image of a page in an e-reader. Persistent display panels have low power consumption while the image is unchanged, which is an important advantage in an e-reader.
A commonly used technology for a persistent display panel is e-paper, which electronically generates an image comparable to a traditional printed ink paper. The image reflects ambient light giving high contrast and a wide reading angle. An example of e-paper technology is an electrophoretic display (EPD), which can hold an unchanging image of text or a picture almost indefinitely with negligible power consumption. An EPD panel has pigmented particles in suspension, usually in micro-capsules between two plates bearing arrays of electrodes. Voltages applied to the electrodes produce electrical fields of pixel size that re-arrange the pigmented particles to update the displayed image.
The image on a persistent display panel can persist for long periods, hours or even days, in the absence of electric fields. However, building up the display of a new image can be slow, and a ghost of the previous image may persist after the frame is updated. A common technique for reducing these concerns is to apply voltages many times during the frame refresh, that is to say the image update.
The successive voltages (waveforms) applied to any given pixel during the frame refresh are a function of the previous state of the pixel, its current transitional state, and its intended new state. Commonly, the waveforms are functions of the temperature of the EPD, which is sensed and serves as a parameter in deriving the waveforms. The waveforms may also be different for different modes of frame refresh, for example initialization, in which any image persisting on the panel is cleared, monochrome, which displays a black-and-white image, gray-scale mode, which is commonly used, and full color mode, which is not widely used yet.
Increasing resolution and screen size of e-reader panels increase the amount of data to process in producing the frame refresh waveforms. Typically, a central processor unit (CPU) extracts pixel data for the image content from memory and loads the relevant pixel data into pixel buffers. A conventional approach uses a dedicated hardware controller that receives the pixel data from the pixel buffers and provides waveform data to drivers that generate the waveforms for the display panel. However, since the hardware controller is specific to the e-reader that it is used with, incompatible applications cannot use it, making a dedicated hardware controller a costly solution.
It is possible to convert pixel data to waveform data using software. However, the quantity of data to be processed, especially for high resolution and large screen sizes makes the frame refresh too slow.
Thus, it would be advantageous to have a controller for a persistent display panel that is less inflexible than a hardware controller, while offering high speed frame refresh.